Am I having a senior moment or what?

What?

Seriously, it looks like either is correct. The word was originally not with an f but with a gh, cognate with German Zwerg. The brick and mortar OED doesn't give a citation in f till the 15th century, so we're out of the OE voiced-between-vowels rule. The OED also gives plurals both dwarfs and dwarves; the former from 1770, 1834 and 1894 and the latter from 1818.

Ya pays yer nickel and ya takes yer cherce.